Type I diabetes is a progressive autoimmune disease that often renders patients dependent on exogenous insulin administration. While therapeutic insulin has greatly enhanced the life expectancy of Type I diabetics, there is significant morbidit and mortality associated with its use. Transplantation of insulin-producing pancreatic islets offers a potential means of restoring normoglycemia independent of exogenous insulin administration. Clinical trials of alloislet transplantation showed tremendous promise in terms of insulin independence at one year after transplant; however, the great majority of patients lost insulin independence by five years post transplantation, most likely as a result of insufficient and/or toxic immunosuppression. Current immunosuppressive agents, in particular calcineurin inhibitors (CNIs), induce significant renal toxicity, adverse cardiovascular events, and can be diabetogenic. There is a clear need for more effective immunosuppressive agents to preserve islet allografts in order to facilitate the further translation of this therapy to clinical practic. Antagonism of T cell costimulation pathways is an effective means of inhibiting alloreactive T cell responses as an alternative to CNIs. Blockade of the CD28 and CD40L pathways with monoclonal antibodies (mAbs) showed tremendous promise in pre-clinical models. Unfortunately, both mAbs are not clinically viable due to unwanted side effects that are the result of Fc-mediated crosslinking. To circumvent these issues, we propose to use novel domain antibody (dAb) costimulation blockers that cannot mediate these adverse events. Data from our lab in skin transplant models has shown that these molecules possess equal or greater efficacy in preventing graft rejection than the existing costimulation blockade therapeutics CTLA-4 Ig and CD154 mAbs. The overall goal of this proposal is to evaluate the potential of these novel dAbs to prevent alloislet rejection in murine models. We will first evaluate the efficacy of the dAbs in fully allogeneic naive and autoimmune T cell models of islet transplantation compared to CTLA-4 Ig and CD154 mAbs. Then, using the OVA TCR transgenic system that has been extensively utilized in our lab, we will investigate the cellular mechanisms of induction of long-term graft survival by these dAbs. The proposed research project will serve as a framework for the applicant's training plan to integrate basic science research on transplantation tolerance into a career as a physician-scientist whose research interest will focus on developing new therapeutics for autoimmune disease.